<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button>点击弹出警示框</button>
    <script>

        // 点击按钮弹出警示框
        var btn = document.querySelector('button');
        btn.addEventListener('click', function() {
            alert('你好，世界');
                });
    </script>
</body>
</html>